1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer that visualizes an electrostatic image having been formed on an image bearing member to obtain an image. More specifically, the present invention relates to an image forming apparatus using a two-component developer including a toner and a carrier as a developer.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a copying machine or a printer employing an electrophotographic printing method, the surface of an electrophotographic photosensitive member (hereinafter simply referred to as “photosensitive member”) acting as an image bearing member is uniformly charged, and thereafter this surface is exposed according to image information. Whereby, an electrostatic image (latent image) is formed on the surface of the photosensitive member. The electrostatic image having been formed on the photosensitive member is developed as a toner image using a developer by a developing device. The toner image on the photosensitive member is transferred to a recording sheet directly or via an intermediate transfer member. Thereafter, by fixing the toner image onto the recording sheet, a recorded image is obtained.
Examples of a developer include a mono-component developer substantially including only toner particles, and a two-component developer including toner particles and carrier particles. A development method of employing the two-component developer is generally advantageous in respect of capable of forming an image of higher definition and good hue or tone.
The two-component developer, in general, is the one in which magnetic particles (carrier) of which particle diameter is about 5 μm to 100 μm, and a toner of which particle diameter is about 1 μm to 10 μm are mixed at a predetermined mixing ratio. The carrier functions to bear a charged toner to carry it to a developing portion. In addition, the toner is charged to be of a predetermined charge amount of a predetermined polarity due to a frictional electrification by being mixed with the carrier.
In the meantime, recently, as an image forming apparatus such as a copying machine and a printer of an electrophotographic method continues to be digitized, to be full-colored, and to have greater processing speed, an output image thereof possesses a value as an original output article, and further much expected to enter into a printing market. Thus, it is required that an image of high quality (high definition) and of a stable image quality can be output. As one of procedures for obtaining an image of high definition, proposed is the method of causing the electrical resistance of the carrier in a two-component developer to be high (Japanese Patent Application Laid-Open No. H08-160671).
That is, normally, in the development method of using a two-component developer, the two-component developer that is borne on a developer bearing member of the developing device is carried to the developing portion opposed to an electrostatic image on the photosensitive member. Then, magnetic brush of the two-component developer on the developer bearing member is made to be in contact or to be close to the photosensitive member. Thereafter, by a predetermined developing bias having been applied to between the developer bearing member and the photosensitive member, only the toner is transferred onto the photosensitive member. Whereby, a toner image corresponding to the electrostatic image on the photosensitive member is formed. On this occasion, when the electrical resistance of the carrier that bears and carries the toner is low, there are some cases where a charge is injected into the electrostatic image through the carrier from the developer bearing member, and thus the electrostatic image is disturbed. When the charge is injected into the electrostatic image, the potential is increased due to the charging of electrostatic image, and thus an image density may become lower.
Incidentally, as a developing bias, an alternate bias voltage in which a DC voltage component and an AC voltage component are superimposed is widely used.
Recently, to enter into the above-described printing market, an electrostatic image of a high resolution has been formed. For example, in the case of 2400 dpi, a dot formation width of 1 dpi is approximately 20 μm, being extremely minute. For example, in the case where an electrostatic image is formed at such a high resolution, the electrostatic image is likely to be largely affected by the charge injection via the carrier from the developer bearing member as described above. Accordingly, it is required to end a development process without damaging such a minute electrostatic image.
Conventionally, as a photosensitive member, an OPC (organic photoconductive) photosensitive member in which a surface protecting layer, a charge transport layer, and a charge generation layer that are made of an organic material are stacked on a metal base, is widely used.
Whereas, to form an electrostatic image of a high resolution as described above, as a photosensitive member, the use of a photosensitive member of a single layer such as an amorphous silicon (non-crystalline silicon) photosensitive member (hereinafter, it is referred to as “a-Si photosensitive member”) is found to be advantageous. One of the reasons can be thought as follows. That is, in the OPC photosensitive member, a charge generation mechanism in an internal part of the photosensitive member is resided in the vicinity of the base of the photosensitive member. Whereas, in the a-Si photosensitive member, the charge generation mechanism in an internal part of the photosensitive member is resided on the surface of the photosensitive member. Therefore, in the case of the a-Si photosensitive member, the charge having been generated in an internal part is not diffused before reaching the surface of the photosensitive member, and thus an electrostatic image of an extremely high brilliance can be obtained.
However, in the case of the a-Si photosensitive member, the surface resistance thereof is low as compared with that of the OPC photosensitive member, the influence of the charge injection via the carrier from the developer bearing member as described above comes to be significantly larger than the case of the OPC photosensitive member. Accordingly, in the case of using the a-Si photosensitive member, a formed electrostatic image is easily to be disturbed. Thus, by setting a higher electrical resistance of the carrier, or causing Vpp (peak-to-peak voltage) of the developing bias to be an alternate bias voltage to be smaller, the transfer amount of the charge is further required to be suppressed.
Here, when causing the VPP of the developing bias to be smaller, although the charge injection via the carrier from the developer bearing member is reduced, the electric field to be exerted on the developer is weakened. Therefore, the force of separating the toner from the carrier is decreased, and thus developability will be reduced. Consequently, to make an image formation of high image quality, it is advantageous to set the electrical resistance of the carrier to be higher.
However, when the electrical resistance of the carrier is made to be higher, the developability, that is the capability of the toner being separated (discharged) from the carrier is found to be likely to decrease.
As described above, the carrier of the two-component developer serves to carry the toner to the developing portion, as well as to provide a charge with respect to the toner by the frictional electrification. Therefore, the carrier is provided with the charge of an opposite polarity to the charging polarity of the toner to be charged. For example, when the toner is charged to be of a negative polarity, the carrier is provided with the charge of a positive polarity.
On this occasion, in case where the electrical resistance of the carrier is high, since the electric charge having been charged in the carrier is hard to transfer, the charge of this carrier and the charge of the toner are attracted each other to be a large attractive force, and thus the toner becomes hard to be separated from the carrier. In case where the electrical resistance of the carrier is low, since the charge in the carrier is likely to diffuse on the surface of the carrier, the attractive force between the toner and the carrier becomes small, and thus the toner comes to be easily separated from the carrier.
FIG. 2 illustrates the difference in developability in the case of using two kinds of conventionally general carriers of different electrical resistance characteristics (low resistance carrier A, high resistance carrier B). In FIG. 2, the abscissa axis represents a peak-to-peak voltage Vpp of the developing bias, and the ordinate axis represents a charge amount per a unit area Q/S [C/cm2] of a toner layer of a toner image formed on the photosensitive member. This Q/S [C/cm2] takes a value obtained by multiplying a charge amount Q/M [μC/g] per a unit weight of the toner of the toner layer on the photosensitive member when obtaining the highest density by a toner bearing amount M/S [mg/cm2] of this toner layer. The above-mentioned Q/S [C/cm2] shows the developability of the developer that is how much of the toner overcomes the attractive force between the carrier and the toner, to be transferred onto the photosensitive member.
Incidentally, FIG. 2 illustrates results in the case of using an OPC photosensitive member of a film thickness (thickness of the photosensitive layer) of 30 μm as a photosensitive member.
FIG. 2 shows that in the case of a large developing bias Vpp, even in the case of high-resistance carrier B, Q/S [C/cm2] equal to that of the low-resistance carrier A can be obtained. Whereas, in the case where the Vpp of the developing bias is low, the electric field for separating the toner from the carrier comes to be small, and thus the developability is found to decrease in the case of the high-resistance carrier B. That is, the attractive force between the toner and the carrier of forces to be exerted on the toner comes to be remarkably large, resulting in the reduction in developability.
In addition, the developability is largely affected by the capacitance of the photosensitive member. When the developability is reduced exceeding the permissible range as the capacitance (capacitance per a unit area) of the photosensitive member is increased, various defective images will be produced. Now, the capacitance of the photosensitive member and the developability will be described.
For example, the case of forming a toner image of the highest density on the following conditions will be thought. A development contrast (potential difference between an image portion potential on the photosensitive member and the DC voltage of the developing bias) Vcont=250 V, a charge amount of the toner Q/M=−30 μC/g, and a toner bearing amount M/S=0.65 mg/cm2. The potential (charging potential) ΔV the toner layer of this toner image forms on the OPC photosensitive member is calculated by the following equation in the case where the film thickness of the OPC photosensitive member is 30 μm.
            Δ      ⁢                          ⁢      V        =                                                      ɛ              t                        ⁢                          ɛ              0                                            2            ⁢            λ            ⁢                                                  ⁢            t                          ⁢                  (                      Q            S                    )                    +                                                  ɛ              d                        ⁢                          ɛ              0                                d                ⁢                  (                      Q            S                    )                                where      ⁢                          (              Q        S            )        =                  (                  Q          M                )            ×              (                  M          S                )            where:Q/M is a toner charge amount per a unit weight on the photosensitive member;M/S is a toner weight per a unit area at the highest density portion on the photosensitive member;λt is a toner film thickness at the highest density portion on the photosensitive member;d is a film thickness of the photosensitive member;εt is a relative permittivity of the toner layer;εd is a relative permittivity of the photosensitive member; andε0 is a vacuum permittivity.
In the case of the above-mentioned conditions, Δ=243 V, and thus Vcont=250 V is charged. That is, it is in the state in which the potential of an electrostatic image is fully charged by the electric charge of the toner layer (charging efficiency of 97%).
On the other hand, an a-Si photosensitive member has material characteristics of about three times larger relative permittivity than that of the OPC photosensitive member (a-Si photosensitive member: about 10, OPC member: about 3.3). Accordingly, the a-Si photosensitive member, in the case of having the film thickness (for example, 30 μm) equal to that of the OPC photosensitive member, is to have the capacitance (for example, 2.95×10−6 F/m2), being three times the capacitance (for example, 0.97×10−6 F/m2) of the OPC photosensitive member.
Considered is the case where a toner image of the highest density is formed on the a-Si photosensitive member on the conditions of Vcont (=250 V) and the charge amount of a toner Q/M (=−30 μC/g) as with the case of the above-mentioned OPC photosensitive member. In this case, from the above-mentioned equation, the toner amount required to satisfy ΔV=250 V is 1.15 mg/cm2, and thus the toner amount of about 1.7 times the toner amount in the case of the above-mentioned OPC photosensitive member is to be transferred onto the a-Si photosensitive member. Conversely, at the developing contrast Vcont of about 1/1.7, the toner bearing amount M/S=0.65 mg/cm2 is to be obtained. Therefore, in the case of the a-Si photosensitive member, at about Vcont=147 V, the electric charge at the high density portion is charged.
However, e.g., in the case of entering into a light printing market, a wide range of tone is required to obtain. Therefore, in case of Vcont=147 V, γ characteristic becomes sharp, and there are some cases where a high tone is hard to obtain.
Furthermore, even in case of an OPC photosensitive member, for the purpose of the sharpness of an electrostatic image, an attempt to reduce the film thickness of the photosensitive member (thickness of the photosensitive layer) has been made. Even in this case, the capacitance of the photosensitive member is increased as the film thickness of the photosensitive member is decreased, the same problems as are described in the above-mentioned a-Si photosensitive member may occur.
To address such problems resulted from a large relative permittivity of the photosensitive member or a small film thickness of the photosensitive member, a method of increasing Q/S [C/cm2] of the toner layer of a toner image, that is increasing the charge amount Q/M [μC/g] of the toner can be thought. For example, the toner charge amount Q/M [μC/g] is set to be −60 μC/g with respect to the above-described −30 μC/g. In this state, supposing that, for example, when the developing contrast Vcont is 240 V, a toner bearing amount M/S [mg/cm2] of 0.65 mg/cm2 can be obtained, ΔV the toner layer forms is to be 238 V (that is about 240 V), and thus the charging efficiency will be about 100%.
However, in actual, when the charge amount Q/M [μC/b] of the toner is increased, since the electrostatic force of the carrier and the toner comes to be exceedingly large, the developability may be largely reduced.
In normal, with respect to a photosensitive member of a large capacitance, in the case of using a high-resistance carrier and a toner of high Q/M, even at a weak electric field the high-resistance carrier forms, the toner is so controlled as to be fully separated from the carrier. That is, with the shape of the toner, an extraneous additive, and further the material of the surface of the carrier, the attractive force (Coulomb force+Van der Waals force+cross linking force) between the carrier and the toner is controlled. However, when the state of the surface of the toner or the carrier is changed due to the performance over a long period, the above-mentioned attractive force may not be controlled.
For example, the toner is extraneously added with a variety of particles (e.g., silica) for controlling a charge amount or fluidity. This extraneous additive also functions as a spacer particle between the toner and the carrier, and largely affects the attractive force between the toner and the carrier. Therefore, for example, in the case where an image output at a low printing ratio continues over a long period, a developer is repeatedly exerted with a shearing force in the developing device, the extraneous additive is embedded in or separated from the surface of the toner, and thus the above-described effect as a spacer may be reduced. As a result, the attractive force between the toner and the carrier will be largely increased. Accordingly, after the image output over a long period, as compared with an initial case, a sufficient developability cannot be ensured, resulting in the possibility of producing e.g., defective images.
For example, in some developers used, while initially M/S=0.65 mg/cm2 has been ensured at the Vcont=240 V, due to the performance with time, only M/S=0.45 mg/cm2 can be obtained at the Vcont=240 V. In this case, the charging potential ΔV with respect to the Vcont is 152 V/240 V≈0.63, that is the potential ΔV the toner layer on the photosensitive member forms just charges about 63% of the Vcont.
Such state in which the potential of an electrostatic image is not charged with the electric charge of the toner can be referred to as “charge failures”. When in this state of “charge failures”, defective images will be produced.
For example, in the case where after the formation of a half-tone image of a low density, a solid image of a high density (image at the highest image density level) is continuously output, when the potential on the side of the high density portion in the developing portion (developing nip) is not charged with the electric charge of the toner, at the boundary portion, a wrap-around electric field from the low-density portion to the high-density portion remains. Since this wrap-around electric field acts to cause the toner on the low-density side to move to the high-density side at the boundary, the so-called “blank area” is generated. That is, “blank are” is the phenomenon that an image comes to be white at the boundary between the low-density portion and the high-density portion. In addition, at the high-density portion, due to the difference between electric field intensities at the edge portion and at the central portion, the so-called “sweep together” phenomenon that the toner is collected at the edge occurs. That is, “sweep together” is the phenomenon that the density at the edge of an image comes to be higher than that at the other portions.
As described above, in the case of a photosensitive member of a low surface resistance, for example, like an a-Si photosensitive member, to faithfully develop an electrostatic image to be formed, desired is a carrier of an electrically high resistance with which no charge injection occurs with respect to the electrostatic image in development. Whereas, with respect to a photosensitive member of a large capacitance such as an a-Si photosensitive member or a thin-film OPC photosensitive member, the increase of the charge amount Q/M [μC/g] of the toner is an effective way for obtaining a sufficient tone with stability without producing defective images such as blank area. However, when the charge amount Q/M [μC/g] of the toner is made higher, developability may be largely reduced. This reduction of developability becomes remarkable as the electrical resistance of the carrier is increased.
With the arrangement, in an image forming apparatus employing a two-component developer including a toner and a carrier, there are some cases where the electrical resistance of the carrier is set to be high in order to prevent the charge injection into an electrostatic image in development, and the charge amount of the toner is increased in order to deal with a photosensitive member of a large capacitance. Furthermore, even in this case, it is desirable not to reduce the developability of the toner charging the potential of the electrostatic image.